Niobium base alloy



United States Patent Oil lice 3,346,379 Patented Oct. 10, 1967 3,346,379 NIOBIUM BASE ALLOY Thor N. Rhodin, Jr., Ithaca, N.Y., assignor, by mesne assignments, to Union Carbide Corporation, a corporation of New York No Drawing. Filed Nov. 15, 1961, Ser. No. 152,676 14 Claims. (Cl. 75-174) This application is a continuation-in-part of my copending application Ser. No. 622,018, filed Nov. 14, 1956, now abandoned.

This invention relates to novel niobium-base alloys and more particularly to improved niobium-molybdenum alloys containing varying amounts of one or more of the elements of the group comprising iron, chromium, tungsten and zirconium; which alloys exhibit unusual strength and oxidation resistance under extreme high-temperature service conditions.

For an alloy to be useful as a material of construction in applications such as jet and diesel engines, atomic power reactors, gas turbines, turbine blades or buckets and nozzle guide vanes for turbines, dies for high-temperature metal working, high-temperature reactors, and the like, it must possess satisfactory high melting point, strength and oxidation resistance properties and also must be amenable to fabrication. Disadvantageously, prior metals and alloys lack these essential qualities to render existent a real need for an alloy capable of satisfactory performance under the service conditions encountered in applications of the type mentioned.

It is among the objects of this invention to overcome these disadvantages of prior metallic construction materials and to provide a novel alloy composition which is particularly adapted and useful for attaining these objects. Further objects of the invention include the provision of an improved, workable alloy composition having superior strength and oxidation resistance characteristics at relatively high, above 1000 C. temperatures, the provision of a niobium-base alloy useful in the applications mentioned and adapted to withstand high mechanical stresses at temperatures above 1000 C.; to provide a niobiummolybdenum base alloy containing other metals, which is desirably resistant to oxidation at temperatures considerably above 800 C., and in a range of 100 C.1300 C. or higher, and which is satisfactorily ductile and readily amenable to mechanical fabrication under hot or cold working or drawing conditions, including hot swaging, hot rolling, forging, extrusion, hot pressing, etc.; to provide a niobium-molybdenum alloy containing other metals, said alloy having requisite, superior hardness properties and which does not require thermal treatment to develop maximum strength at elevated temperatures; to provide an alloy composition of the type mentioned having superior fatigue, tensile and rupture properties at relatively high temperatures and which undergoes no significant permanent dimensional changes upon subjection to prolonged exposure under extreme temperature conditions; and to provide a niobium-molybdenum alloy composition containing other elements and having unique, especially protective surface layers of reaction products and which advanta eously exhibits such chemical and mechanical attributes as desired adherence, non-permeability, dimensional stability, nonvolatility and minimum film thickness when exposed to corrosive atmospheres at high tempera tures. Other objects and advantages of the invention will be evident from the ensuing detailed description.

These and other objects are realized by the alloys of this invention which contains as essential ingredients at least 55% by weight of niobium, about 120% by weight of molybdenum, and 130% of at least one element selected from the group comprising iron, chromium, tungsten and zirconium, the total or" the elements of this group not to exceed 35%. In combination with these elements, and to impart to the alloy certain desired characteristics, such as the properties of protective oxide scale or the special metallurgical response of the alloy to working, such as for example, heat treatment or fabrication, there may be added from 0-5% by weight of one or more of the elements beryllium, manganese, silicon, thorium, and vanadium; and from 02% by weight of one or more of the elements boron, carbon, calcium and cerium. When mixtures of two or more of such added elements are employed from said 05% range, the total should not exceed 15% by weight; and when from said 02% range, the total should not exceed 5% by Weight.

In a more specific and preferred embodiment, my novel alloy composition contains from about 55% to by weight of niobium, and from 8% to 20% by weight of molybdenum, with the amounts of added elements present being in the following ranges and total amounts: 1% to 30% by weight of one or more of the elements iron, chromium, tungsten and zirconium, with the total of this group not to exceed 35%; 05% by weight of one or more of the elements beryllium, manganese, silicon, thorium and vanadium, the total of this group not to exceed 15 and from 02% by weight of the elements boron, carbon, calcium and cerium, the total of this group not to exceed 5%. If desired, the composition can contain from about 14% of iron.

My improved alloys can be prepared in accordance with conventional procedures and through recourse to known melting and casting techniques. Thus, the individual metals can be melt cast together and the melt allowed to cool and solidify into a desired shape. The melting operation can be carried out in an arc melting furnace provided with consumable or non-consumable electrodes, or through subjection of the charge to induction heating in a skull or crucible type of container. One useful form of arc melting furnace comprises that having an integral, Water-cooled copper crucible in which the charge can be melted and solidified, such as that described by W. Kroll in Transactions of the Electrochemical Society, vol. 78, pages 35-37, 1940. Alternatively, a compressed, consumable arc electrode type melting furnace can be employed, such as described by S. A. Herres, U.S. 2,640,860, as can the combination of a non-consumable and consumable electrode type of double melting furnace described in U.S. 2,541,764, to S. A. Herres. A continuous feed type of furnace can also be used, such as described in U.S.P.B. Report 111,083. Whatever type furnacing means is employed, care should be exercised in the melting and casting operation to protect the molten metal from normal atmospheric contamination through contact with oxygen, nitrogen, etc. This can be prevented by conducting the operation under -a vacuum or an atmosphere of an inert gas, such as argon, helium, etc.

The individual metals charged to the melting furnace can be in any desired form, e.g., powder, granular, shot, wire or sponge, and should be of commercially acceptable purity to insure production of a satisfactory pure alloy product. The cast material obtained will consist of a workable metal having excellent strength and oxidation resistance at high temperatures, and will be eminently suitable for use as a material of construction in high tem perature equipment designed to operate at temperatures beyond the limits of present equipment constructed of the best high-temperature alloys.

Advantageously, my alloys will exhibit :high strength at temperatures ranging from 1000 C.1300 C. or higher, at which temperatures other high-temperature alloys lose strength, become plastic or melt; will be characterized by especially protective layers of reaction products on or at the metal surface consisting of compounds of the alloy with oxygen, nitrogen, hydrogen, carbon,

sulfur or halogens or compounds thereof present in the atmosphere; and will be found to have been adjusted to produce especially protective surface layers containing combinations of the compounds mentioned with themselves, or with each other, such as mixed oxides-nitrides, to provide very high resistance to deleterious attack of the alloy by the surrounding gases. The data given below demonstrate the properties of the alloy in respect to ductility and high-temperature oxidation resistance. Their performance with reference to a balance between oxidation resistance and fabricability is determined by the relative proportions of the alloying elements. Since these two properties tend to oppose each other, the ranges in composition given were chosen on the basis of establishing an optimum compromise between them.

To a clearer understanding of the invention, the following specific examples are given. These are only illustrative and are not to be considered as limiting the scope and underlying principles of the invention. As will be noted, the niobium-base alloys given are characterized by high niobium content and in the preferred 55-80% range. In addition, the protective films characteristic of oxidation at 1000 C. and 1200 C. often contain relatively little niobium oxide.

Example I A granular mixture of 58% by weight niobium, 20% by weight molybdenum, and 22% by weight of chromium, is charged into a water-cooled, copper crucible of an arc melting furnace of the type described above and the metals heated under an atmosphere of helium to effect complete fusion of the metal charge. When the charge became liquefied, the furnace was turned off, and the melt was allowed to cool in the inert atmosphere, discharged from the crucible, and was tested for resistance to high-temperature oxidation in the following manner:

A coupon was cut from the as cast button and heated at 1000 C. and 1200 C. for 24 hours in a helium atmosphere. The sample was then heated to 1000 C. and 1200 C. in a recording thermobalance in flowing air for 24 hours. Oxidation rates were followed by making continuous measurements of weight change while the sample was at a controlled temperature without interruption of the test during the 24-hour period. Non-volatility of surface compounds under these test conditions was also determined by measuring no Weight change when exposed to pure helium.

Upon termination of the oxidation test, the sample was cooled and the protective character of the surface layers determined by metallographic examination and chemical analysis. In addition, the effect of oxidation of the metal alloy itself was examined by the same methods. It had an oxidation rate of 0.03 mg./sq. cm./hr. after 24 hours at 1000 C. and a rate of 0.08 mg./sq. cm./hr. after 24 hours at 1200 C. A specimen of pure niobium metal, subjected to the same test, in contrast, had an oxidation rate of 22.0 mg./-sq. cum/hr. after 24 hours at 1000 C. and 68 mg./sq. cm./hr. at 1200 C.; and, in some cases, was completely converted to the oxide after treatment at 1000 C. and 1200 C. The specimen of this example, on the other hand, was covered with a very thin, especially adherent protective oxide layer which corresponded to less than 0.05% conversion to the metal at 1000 C. and less than 0.12% conversion at 1200 C. This layer showed outstanding resistance to spalling when the specimen was heated to 1000 C. and 1200 C. and then cooled to room temperature.

Upon forging and machining the remaining casting into a nozzle element and employing such nozzle for spraying MgCl at a temperature above 800 C. in a chemical process, the alloy exhibited excellent high-temperature oxidation resistance characteristics and proved to be highly useful in such applications.

4 Example 11 An alloy was prepared as in Example I except that its composition was 61% by weight niobium, 13% by weight molybdenum and 26% by weight chromium. Upon subjecting a coupon cut from the as-cast material to the test described in Example I, it had characteristics as shown in Table I below.

Example 111 An alloy was prepared as described in Example I except that its composition comprised 57% by weight niobium, 20% by weight molybdenum, 20% by weight chromium and 3 by weight tungsten. Upon testing as in Example I, it had the properties shown in Table I below.

Example IV An alloy Was prepared as described-in Example I except that its composition comprised 56% by weight niobium, 20% by weight molybdenum, 14% by weight chromium, and 10% by Weight zirconium. Its characteristics are shown in Table I below.

An alloy composition containing niobium, 8% molybdenum, and 2% iron was prepared in accordance with the procedure described in Example 1. Three coupons x x were cut from the as cast button obtained, were carefully measuredgand were then tested for their resitsance to oxidation in flowing air (2-4 cu. ft. per hour) at 1000 C. for hours. After cooling, the specimens were cut in half and their cross sections measured. The depth of recession as shown in Table 11 below, comprises the extent to which the oxidizing atmosphere penetrated the face of the specimens and converted the alloy to external scale. For convenience, measurements were made from either of the large faces. The depth of recession on either of these faces was substantially the same. Also, the cross sectional area of the sample was examined microscopically to determine the degree of internal oxidation which might have occurred. No internal oxidation was found to exist.

Example VI An alloy composition containing 88% niobium, 8% molybdenum and 4% iron was prepared as in Example V, and three samples of the alloy product were tested for oxidation resistance. Surface recession measurements and internal oxidation results for this alloy are also shown in Table 11 below.

TABLE II.OXIDATION TESTING OF Nb-Mo-Fe ALLOYS 1 Repeat.

In addition to the foregoing, the alloy compositions shown in Table III below were prepared in accordance with the melting procedure of Example I. These were found to exhibit the valuable high-temperature strength properties shown in Table III below. Each specimen tested was machined from the as cast buttons, was an nealed 1 hour at 1200 C., and was tested for ultimate tensile strength, 0.2% yield strength, and percent elongation. All tensile tests were carried out in accordance with A.S.T.M. Designation E21-58T specification. By 0.2% yield strength is meant the stress at which the alloy sample exhibits a 0.2% deviation from direct relationship of stress to strain. By percent elongation is meant the degree of permanent extension before fracture, expressed as a percentage of the original gauge length.

Alloys with diamond pyramid numbers in excess of 75 at temperatures greater than 800 C. are generally accepted as showing good high-temperature strength.

As noted, the alloys of this invention are useful as materials of construction in all applications requiring strength and a corrosion-resistant metal. Hence, while particularly useful in high-temperature equipment which TABLE Ill-STRENGTH PROPERTIES OF NbZr-Mo-C ALLOYS TEST- ED AT 1200 C.

Alloy Composition Ultimate Percent Tensile 0.2% Yield Elongation Strength Strength (1" Nb Mo Zr (p.s.i.) (p.s.i.) gauge) percent percent percent 1 None added Example VII An alloy composition containing 93% niobium, 6% molybdenum, and 1% iron was and prepared tested for high temperature strength in accordance with the procedures shown for the alloys of Table III. The tensile specimen was prepared by extruding the as cast specimens and at a 4:1 ratio at about 1370 C. and then re-extruding at a 6:1 ratio at about 17 70 C. The high-temperature strength of these specimens is shown in the following Table IV.

must operate above 800 C., such as jet engine parts, nuclear reactors, gas turbine parts, etc., my novel alloys due to their outstanding properties, including nonbrittleness and adaptability for successful fabrication by hot swaging or rolling, forging or extrusion, hot pressing, are not restricted to such applications nor to any particular equipment described or referred to herein.

When recourse to beryllium, manganese, silicon, thorium and vanadium use in combination with alloying TABLE IV.STRENGTH 0F EXTRU IPED ALLOY: 93% Nb, 6% M0, 1%Fe TEST- *R.A. indicates the percentage reduction of the cross-sectional area at the fracture as compared with the original crossseot1onal area of the test piece.

Example VIII An alloy composition containing 70% niobium, 6% molybdenum, and 24% tungsten was prepared by melting in accordance with the procedure shown in Example I. A specimen of this alloy was stress-rupture tested at 1200" C. and 1370" C., such test being performed at constant load and constant temperature, and the load being held at such a level as to cause rupture of the specimen. At 1200 C., a load of 25,000 psi. caused rupture of the specimen in 50.8 hours. At 1370 C., a load of 15,000 p.s.i. caused rupture of the specimen in 146 hours.

Example IX An alloy composition containing 93% niobium, 6% molybdenum, and 1% iron was prepared by the melting procedure described in Example I. The as-cast alloy was machined to 3" ingot size and was then extruded at about 1370 C. at a 4:1 ratio and re-extruded at about 1790 C. at a 6:1 ratio. The resulting product was then tested for hardness at the temperatures shown in Table IV below, While employing a diamond pyramid hardness test. Since it is an accepted metallurgical fact that the hardness of metals is directly correlated with their strength (see R. T. Rolfe, Dictionary of Metallurgy, page 247), the results shown in such table demonstrate further the valuable elements mentioned is had, amounts from 0.15% by weight can be resorted to, as can from 0. 12% by weight of the elements boron, carbon, calcium and cerium.

Although I prefer to employ metals exhibiting relatively high purity, some variance in purity properties can be tolerated. Thus, the alloys of the examples and those tested were obtained from commercially available niobium and molybdenum containing less than 1% incidental impurities. Com-merical niobium usually contains tantalum (in amounts up to 5%) which is difficult to detect and separate. Therefore, the niobium contemplated for use herein may contain small amounts (0.1 to 5.0%) of tantalum as well as oxygen and possibily silicon as impurities. Elimination of certain of these impurities (oxygen) or enhancement of others (tantalum) may improve oxidation resistance significantly.

Since many changes and modifications can be made in the invention without departing from its underlying principles, it will be understood that it is not restricted to the above detailed description, but only as defined in the appended claims.

I claim:

1. A niobium-base alloy consisting essentially of at least 55% by weight of niobium, from 1-20% by Weight of molybdenum and from 1-30% by weight of at least one element selected from the group consisting of iron,

7 v chromium, tungsten and zirconium, the total of the elements selected from this l30% group not to exceed a total of 35%, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

2. An oxidation-resistant niobium-base alloy composition consisting essentially of from about 55a80% by weight of niobium, from about 1 20% by weight of molybdenum, and from about l30% by weight and not to exceed 35% by Weight in total of at least one element selected from the group consisting of iron, chromium, tungsten and zirconium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

3. A niobium-base alloy consisting essentially of at least 55% by weight of niobium, from l-20% by Weight of molybdenum, from 130% by weight of at least one element selected from the group consisting of iron, chromium, tungsten and zirconium; and to which may be added, if desired, up to 5% by Weight of at least one element selected from the group consisting of beryllium, managanese, silicon, thorium and vanadium, and up to 2% by weight of at least one element selected from the group consisting of boron, carbon, calcium and cerium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

4. An oxidation-resistant nobium-base alloy composition consisting essentially of from about 5580% by weight of niobium, from about 120% by weight of molybdenum and from about l30% by Weight of chromium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

5. An oxidation-resistant niobium-base alloy composition consisting essentially of from about 55-80% by weight of niobium, from about 120% by weight of molybdenum, and from about .l30% by weight of zirconium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

6. An oxidation-resistant niobium-base alloy composition consisting essentially of from about 55-80% by Weight of niobium, from about l20% by weight of molybdenum, and from about 130% by weight of tungsten, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

7. An oxidation-resistant nibium-base alloy composition consisting essentially of from about 5580% by weight of niobium, from about 120% by weight of molybdenum, and from about l30% by weight of iron, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

8. An oxidation-resistant niobium-base alloy composition consisting essentially of from about, by weight, 55-80% of niobium, from about l20% molybdenum, and from about 14% of iron, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

9. An oxidation-resistant alloy consisting essentially of, by weight, from about 58% niobium, 20% molybdenum, and 22% chromium.

10. An oxidation-resistant alloy consisting essentially of about, by weight, 56% niobium, 20% molybdenum, 14% chromium and zirconium.

11. A niobium base alloy consisting essentially of at least 55% by weight of niobium, from 120% by weight of molybdenum, from l 30% by weight of at least one element selected from the group consisting of iron, chromium, tungsten, and zirconium, the total of the elements of this 130% group not to exceed 35%; and to which may be added, if desired, up to 5% by weight of at least one element selected from the group consisting of beryllium, manganese, silicon, thorium, and vanadium, the total of the elements of this up to 5% group not to exceed 15%; and up to 2% by weight of at least one element selected from the group consisting of boron, carbon, calcium and cerium, the total of this up to 2% group not to exceed 5%, said alloy being adapted to Withstand prolonged exposure at a temperature above 800 C.

12. A niobium-base alloy composition consisting essentially of at least 55% by Weight of niobium, from l20% by weight of molybdenum and from about 130% by weight of zirconium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

13. A niobium-base alloy consisting essentially of at least 55 by weight of niobium, from 120% by Weight of molybdenum, from 130% by weight of zirconium and up to 2% by weight of at least one element selected from the group consisting of boron, carbon, calcium and cerium, with the total of said group not exceeding 5% by weight, said alloy being capable of withstanding prolonged exposure at a temperature above 800 C.

14. A ternary alloy consisting essentially of by weight from 120% molybdenum, from .1-30% tungsten, the sum of the contents of molybdenum and tungsten being from 2-45%, and the balance coilumbium in an amount of from 55 to 98%.

References Cited UNITED STATES PATENTS 1,588,518 6/1926 Brace 74-174 2,822,268 2/ 1958 Hix 75-174 2,838,395 6/1958 Rhodin 751174 2,881,069 4/ 1959 Rhodin 75l74 2,882,146 4/1959 Rhodin 75l74 2,973,261 2/21961 Frank 75l74 3,046,109 7/ 1962 Lottridge 75l74 FOREIGN PATENTS 1,190,580 4/ 1959 France.

OTHER REFERENCES Initial Investigation of Niobium and Niobium-base Alloys, Saller et al., US. Atomic Energy Commission Report BMI l003, May 23, 1955, page 43.

I-IYLAND BIZOT, Primary Examiner.

RAY K. WINDHAM, WINSTON A. DOUGLAS,

Examiners.

DAVID L. REC-K, BENJAMIN HENKIN, W. B. NOLL,

W. C. TOWNSEND, C. N. LOVELL,

Assistant Examiners. 

1. A NIOBIUM-BASE ALLOY CONSISTING ESSENTIALLY OF AT LEAST 55% BY WEIGHT OF NIOBIUM, FROM 1-20% BY WEIGHT OF MOLYBDENUM AND FROM 1-30% BY WEIGHT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF IRON, CHROMIUM, TUNGSTEN AND ZIRCONIUM, THE TOTAL OF THE ELEMENTS SELECTED ROM THIS 1-30% GROUP NOT TO EXCEED A TOTAL OF 35%, SAID ALLOY BEING ADAPTED TO WITHSTAND PROLONGED EXPOSURE AT A TEMPERATURE ABOVE 800*C. 